Oral administration of pharmaceutical compositions still remains a route of choice for most clinical applications. Some drugs have ideal characteristics for good absorption throughout the gastrointestinal tract (GIT) while others present shortcomings in this regard.
In drugs having such shortcomings, the oral route poses a challenge for pharmaceutical compositions that display site-specific absorption. Often such compounds demonstrate poor and variable bioavailability as a result of their small site for absorption within the GIT. These pharmaceutical compositions are said to have a “Narrow Absorption Window” (NAW).
The transit rate of a pharmaceutical dosage form through the GIT determines the time that it remains in contact with its preferred site of absorption. In humans, the transit time in the stomach ranges between one to two hours, in the small intestine remains fairly constant at three hours while, in the colon, this could be as long as twenty hours. Accordingly, for pharmaceutical compositions absorbed in the intestine, the relatively short residence time promotes absorption in the proximal intestine rather than the distal regions. The gastric time determines the duration that pharmaceutical compositions remain in contact with its specific site of adsorption and, therefore, the bioavailability may be enhanced by prolonging the transit time of a pharmaceutical dosage form in the gastrointestinal tract.
Prolonging the release of pharmaceutical compositions within the gastrointestinal tract by way of increasing gastric residence time offers numerous advantages over conventional oral immediate-release drug delivery system as the pharmaceutical composition is released into the stomach and intestines over a longer period of time, allowing more time to be available for pharmaceutical compositions with low bioavailability or narrow absorption windows to be absorbed. Furthermore, free-drug is available for absorption at the “narrow absorption window” region as result of the pharmaceutical dosage form retained for longer in the gastric region with subsequent release of drug before entering the intestine where majority of the “narrow absorption window's” for such drugs are found.
There are numerous factors which affect gastric emptying and, as a result, may influence the gastric retention time of a pharmaceutical dosage form. The size and shape of such a pharmaceutical dosage form affects the transit through the pyloric sphincter, while its density determines its gastrofloatibility (resulting in buoyancy on gastric contents) or gastroimmensity (resulting in sinking toward the antrum of the gastric region). These factors are important to consider when designing a GIT drug delivery system.
Biological factors also play an important role in the functioning of the GIT and, consequently, the uptake kinetics of an orally administered pharmaceutical compaction. These biological factors include the age and gender of the patient, the presence of disease as well as their level of physical activity, body mass index and posture. Further factors that influence gastric emptying include the ingestion of food and particular drugs which may have an impact on GIT motility.